Breathable microporous film and methods for making it

ABSTRACT

A soft, flexible, microporous film is provided having high tensile strength and good air and water vapor transmission rates therethrough while being substantially impenetrable to liquid water. This film is prepared by stretching a casting of a composition of a polyolefin and high levels of CaCO 3  or glass beads and calcium stearate in two directions from 1.5-7 times in each direction to produce a film having a Gurley porosity of from 0.1 to 85 seconds. This film is especially useful for disposable items such as feminine panty liners, diapers, bed sheets, and hospital gowns. When glass beads are substituted for the CaCO 3 , the film is useful for battery separators.

BACKGROUND OF INVENTION

This invention relates to soft, flexible, microporous films having hightensile strength and good air and water vapor transmission ratestherethrough but which are substantially impenetrable to liquid water.This invention also comprehends a process for making such films bystretching certain highly filled polymer compositions under controlledconditions.

Prior to the present invention, breathable microporous film were made bya variety of techniques from polymers containing solid substancesdispersed therein. Sometimes the pores of the film were obtained bydissolving or leaching out those dispersed materials (fillers) andsometimes Pores were formed when the filled polymer material wasstretched. Often the porous films produced by stretching the filledpolymer compositions were stiff, low tear strength products,irrespective of the type of matrix polymer used. Hence, there is a needin the hygienic product industry for liquid barrier yet breathable,disposable products such as in feminine panty liners, diapers, bedsheets, and hospital gowns. In order for public acceptance of breathableproducts, the user should be cool and comfortable and the product has tobe soft and flexible without irritating the skin of the user. There isalso a need for such breathable films in the battery industry for use asbattery separators. The present invention solves the above-mentionedneeds.

U.S. Pat. No. 4,698,372 discloses a microporous Polymeric film havinggood water vapor transmission rates and hydrostatic resistance to waterpenetration thereof; the film has a filler loading of about 25-35 volume% of inorganic fillers such as calcium carbonate, among others, and usesan "antagonizer" such as stearic acid in order to reduce the effectivesurface tension of the filler to the approximate level of that of thematrix polymer. U.S. Pat. No. 3,903,234 discloses gas permeablebiaxially oriented film prepared from compositions of polyolefinscontaining 26-50% by weight of inorganic filler particles. U. S. Pat.No. 4,176,148 discloses microporous oriented films composed ofpolybutene containing 3-80% by weight of inorganic fillers. BritishPatent 2,151,538 discloses a process for making water vapor permeableoriented film from polyolefins containing 33-83% by weight of bariumsulfate filler.

None of the above prior references discloses the instant invention.

SUMMARY OF THE INVENTION

This invention is directed to a breathable film composition comprising:

20 to 37% by weight of a polymer or copolymer of an α-olefin having 1-8carbons or mixtures thereof,

60 to 75% by weight of calcium carbonate or glass beads having particlesizes of 10 to 15 micrometers,

0.1 to 3% by weight of calcium stearate, and

optionally, 0 to 2% by weight of a stabilizer, wherein moisture level inthe blended composition is maintained below 700 ppm prior to forming acasting and then, based on the polymer or copolymer used, stretching thecasting in 2 directions from about 1.5 to about 7 times in eachdirection in a temperature range of from about 20° to about 160° C. andeach of the films having a Gurley porosity (based on method B, ASTMD-726) of 0.1 second to 85 seconds so that the film has good air andwater vapor transmission rates but is substantially impenetrable byliquid water.

This invention also comprehends the method of preparing the breathablemicroporous polymeric film composition, mentioned above, by maintainingthe moisture level below 700 ppm (preferably below 300 ppm) in theblended composition prior to forming a casting and then, based on thepolymer or copolymer used, Stretching the casting in two directions fromabout 1.5 to 7 times in each direction in a temperature range of fromabout 20° to about 160° C.

DETAILED DESCRIPTION OF THE INVENTION

For the purpose of this invention, a high "breathability" is synonymouswith low Gurley porosity number measured in seconds by ASTM D-726,Method A or Method B. ASTM D-726, Method A, measures the time (inseconds) for 100 milliliters of air to pass through one square inch ofmicroporous film under a pressure of 4.9 inches of water. ASTM D-726,Method B, measures the time (in seconds) for ten milliliters of air topass through one square inch of microporous film under a pressure of12.2 inches of water. Theoretically, Gurley numbers measured by Method Aare 25 times larger than Gurley numbers measured by Method B. A lowGurley number signifies that a microporous film offers little resistanceto the passage of air (or humid air). Thus, Gurley number (Gurleyporosity) is a simple measure of "breathability".

"Microporous" means that the film contains numerous open pores orchannels leading from one surface to the opposite surface, such poresbeing of a size to permit air and water vapor to pass through the filmwhile having good resistance to the penetration of liquid water. Thepermeance of the microporous film of this invention is greater than30,000,000 cc/100in -day-atmosphere (or 465,000,000 cc/m²-day-atmosphere).

In the present invention, a variety of materials can be selected asmatrix polymers. Generally, the polyolefins are preferred. The selectionof the polymeric material will be based on the desired properties of themicroporous film, as for example, temperature resistance or elasticrecovery. Thermoplastic, orientable polymeric materials which exhibittensile yielding and some permanent deformation may be used. Examples ofhomopolymers which can be used in this invention are polypropylene (PP),polyethylene (PE), polybutylene (PB), and 4-methylpentene. Examples ofcopolymers are copolymers of ethylene with propylene or an α-olefin of4-8 carbons. Processing aids, preferably calcium stearate, coat thefiller particles, thus assisting in the uniform dispersion of the fillerparticles thereby allowing the composition to be stretched to a highdegree of orientation.

In the polyproPylene/calcium carbonate system, the amount of the calciumcarbonate filler should be in the range of about 60 to 70% by weight ofthe composition, preferably 65%. The amount of calcium stearate shouldbe in the range of 0.1 to 3.0% by weight, preferably 0.5 to 2.0%. Thebiaxial orientation of the film should be in the range of from 4 to 7times in each direction, preferably 5 times, with the orientationtemperature being from 130° to 150° C., with 130° C. being preferred.

In the polypropylene/glass beads system, the amount of the filler shouldbe in the range of 55 to 65% by weight of the composition, preferably65%. In this system the amount of calcium stearate should be 0.1 to 3.0%by weight, preferably 0.5 to 2.0%. This film should be biaxiallyoriented by stretching from 4 to 7 times in each direction, with 5 timesbeing the preferred stretching, at a temperature of from 130 to 150° C.,preferably 135° C.

In the polybutylene/calcium carbonate system, the amount of the fillershould be in the range of 65 to 75% by weight of the composition, with70% being preferred; in this system the amount of calcium stearate to beused should be in the range of 0.2 to 4% by weight, with 2% beingpreferred. This film should be biaxially oriented from 1.5 to 5 times,preferably 4 times, at a temperature range of 20° to 105° C., 100° C.being preferred.

In the polyethylene/calcium carbonate system, the amount of fillershould be in the range of 60 to 70% by weight, 70% being preferred. Theamount of calcium stearate in this system should be from 0.1 to 3.5%,0.5 to 2% being preferred. This film should be biaxially oriented 1.5 to5 times, with 4 times being preferred, in a temperature range of 20° to110° C., with 100° C. being preferred. A blend of linear low densitypolyethylene and polypropylene in a ratio of 95 to 5 can improveprocessibility by reducing die line problems compared to the use oflinear low density polyethylene (LLDPE) alone.

A blend of polypropylene with ethylene-propylene copolymer can beprocessed more readily than a similar composition which containspolypropylene as the sole polymer. When glass beads are used instead ofthe calcium carbonate as the filler, the films are useful as batteryseparators.

The particle size of the filler ultimately determines the pore size ofthe microporous films of this invention. Thus, smaller particle sizes offillers permit attainment of smaller pores compared to larger particlesizes of fillers at equal loading and equal overall porosity. There isno theoretical limitation on the size of the fillers which may be usedin the practice of this invention. However, practical considerationsimpose effective limitations. In the present invention, it has beenfound that fillers of particle size ranging from 10 to 25 micrometers inmean diameter are preferred over smaller particle size fillers in orderto attain the high breathability of the instant film. The preferredparticle size of the calcium carbonate filler is about 12.5 micrometersmean diameter.

Filler loading determines to a great extent how far the precursor filmmust be stretched to attain a given degree of overall porosity. Belowthe lower end of the loading range, the pores are less numerous and lessinterconnected; therefore, the film is less permeable at a given drawratio than when a higher filler loading is employed. Above the higherend of the loading range, either the materials will not blend uniformlyor the casting made from the composition will not stretch. The preferredloading in the present invention is about 60 to 75% by weight of thecomPosition, preferably 65% to 75%. Although other inorganic fillers maybe used, calcium carbonate is preferred.

Stabilizers are usually used in the composition of the present inventionin the range of from 0.1 to 2% by weight as a means to insurestabilization of the system to UV light, oxygen, and heat; it isespecially useful in the systems with the polypropylene and thepolyethylene.

A critical requirement in the process of this invention is to maintainthe moisture level in the composition below 700 ppm prior to extrudingthe casting so as to create a casting with a smooth surface. A smoothsurface is necessary to enable the casting to be stretched uniformly.

After the film composition is prepared, it may be compounded into thefilm of this invention by any known method suitable for the meltblending of thermoplastic polymers at temperatures at which the matrixpolymers are processible. High shear mixing, which can be achieved in aBanbury-type or another high intensity mixer or in continuous mixerssuch as extruders, is preferred. There is no need to premix ingredients,but this may be done without detriment to the practice of this inventionand may in certain instances offer improved performance.

After the ingredients of the composition of this invention have beenmelt blended, the moisture level of this blend is then maintained below700 parts per million (ppm) (preferably below 300 ppm). A preferredmethod for maintaining the moisture content at the desired levels is tocool extruded strands on a moving conveyor belt using flowing air. Thisair-cooling method yields strands and pellets which have residualmoisture levels far below the levels achieved by the water-bath-coolingprocess typical in the industry.

The strands were then pelletized using conventional techniques in theindustry. To accurately achieve this moisture level, sensitive moisturemeasurement techniques are required. For example, a Coulometric KarlFischer titration method (using the Brinkman Model 652 RF Coulometer)was used successfully for verifying the desired moisture level in theformulations.

After blending and maintaining the moisture level, the composition isconverted into any convenient form for processing into film, includingpellets or sheets. The film fabrication ca be accomplished by anyconvenient technique including compression molding, flat film extrusion,or blown film extrusion.

After the film is fabricated into its desired form, it is then biaxiallyoriented by stretching by any of the well known techniques in the artincluding, by hydraulics, by pinch rolls moving at different rates, orby tentering. Biaxial stretching can be performed sequentially orsimultaneously. Sequential biaxial stretching is preferred when usingthe tentering operation.

Another process of maintaining the desired moisture level is to emploYvacuum-drying in order to reduce the moisture level in too-wet pelletsto acceptable levels (below 700 ppm, and preferably below 300 ppm). Inthis case, pellets composed of polymer plus filler would be made using awater-bath-cooling process such that the residual moisture level isexcessive. These too-wet pellets can be subjected to a partial vacuum,preferably with some heating to speed the process, for a period of timeuntil the moisture content is within acceptable limits as defined above.This process works but is not the preferred one since an extra process,vacuumdrying, is required.

Yet another process of maintaining the desired moisture level is bycharging the hot melt directly to the extruder which extrudes thecasting from a die. In this case, the molten composition is neverexposed to liquid water and, thus, has a low residual moisture level asdefined above. Therefore, a smooth and highly-orientable casting will beformed.

The stretch ratio of at least two times the original forming dimensionsis significant to producing a film having at least 30% of poresresulting in relatively high density films. However, to producerelatively low density films, it is preferred that the film be stretchedto at least 3 to 8 times its original forming dimensions in mutuallyperpendicular directions, resulting in a film having about 40 to 70%pores.

Stretching is effected above the glass transition temperature of thematrix polymer (preferably at least 20° C. above) and below the meltingtemperature of the matrix polymer, especially within 10° C. of thattemperature, depending to some degree on the rate of stretching.Different polymers and compositions thereof exhibit different elasticand viscoelastic behavior. Thus, different amounts of stretching must beimposed on different samples in order to obtain the same permeabilityproperties. Nevertheless, the film must be stretched beyond its yieldpoint in order to attain the permanent deformation necessary for theformation of porosity.

For a given composition, a greater degree of stretch results in greateroverall porosity. Higher overall porosity can be attained by adding morefiller and stretching, the same amount or less.

In the following examples, all parts, proportions, and percentages areby weight unless otherwise indicated.

Examples 1-14

In Examples 1-8, the ingredients (listed in Table 1) were blended atroom temperature and compounded in a twin-screw extruder; strands wereextruded at in a temperature range of 243° to 265° C. The strands werethen air cooled (except in Examples 6-8 that were water cooled) andpelletized. The pellets were vacuum dried for 24 hours at 80° C. (exceptExamples 7 and 8 that were vacuum dried for 8 hours at 70° C. Using amelt temperature of 478° to 540° F., the pellets were extruded by asingle screw extruder through a six inch wide slit die onto a castingroll maintained at about 65° C. (except Examples 6-8 were maintained atabout 18° -24° C.) so as to form a 15 mil thick casting. Using a T. M.Long stretcher, square pieces having the dimensions 2×2 inches from thecasting were biaxially oriented by stretching 4 times in the machinedirection and 4 times in the transverse direction (except Example 8 wasstretched 2× by 2×) at 100° C., producing the product as set forth inthe following Table 1.

In Examples 9 and 10, the ingredients were blended together on a 2-rollmill at 200° C.; this blend was compression molded at 215° C. to yield30 mil thick plaques. Two inch by two inch portions of the plaques werebiaxially oriented by stretching 5 times in the machine direction and 5times in the transverse direction on a T. M. Long stretcher at 140° C.to make the film as described in Table 1.

In Examples 11 and 14, the ingredients were compounded in a twin screwextruder at 225° -250° C.; the extrudate was pelletized and cast on acasting extruder at 180° -230° C. For Example 11 since much strandbreakage and non-uniformity was observed during the pelletizing step,the casting could not be stretched at 140° C. on the T. M. Longstretcher; the casting was too brittle. For Example 14, 2×2 inchportions of the casting were stretched 4.5× by 4.5× at 140° C. on the T.M. Long stretcher.

In Example 12 and 13, the ingredients were blended by a twin-screwextruder and were extruded by a single screw extruder and slit die toform a 30 mil casting; the casting was stretched 5× by 5× on a T. M.Long stretcher to form the film.

    TABLE 1       1a,b,c, 2 3 Com 4 5 6 Com 7 Com 8 9 Com 10 11 12 13 14       POLYMER LLDPE LLDPE LLDPE LLDPE LLDPE/PP PB PB PB PP/C.sub.2 C.sub.3 *     PP/C.sub.2 C.sub.3 PP PP/C.sub.2 C.sub.3 PP/C.sub.2 C.sub.3 PP (%)       (1:1) (1:1)   (1:1) (1:1)  28.5 33.25 38.0 29.4 27.0/1.5 28.5 50.0     30.0 39.83 49.74 39.64 34.61 34.91 36.05 CALCIUM 1.5 1.75 2.0 0.6 1.5     1.5 0 0 0.32 0.40 0.40 1.04 0.17 1.09 STEARATE (%) CaCO.sub.3  (%) 70 65     60 70 70 70 50 70 59.75 49.74 59.46 64.27 64.83 -- STABILIZER     0.10 0.12 0.50 0.09 0.09 0.16 GLASS BEADS (%)              62.71       PRODUCT PROPERTIES   1a,b,c, 2 3 Com 4 5 6 Com 7 Com 8 9 Com 10 11  12 1     3 14       ORIENTATION/ a 4× by 4×/ 4× by 4×/ 4× by     4×/ Mat'ls. 4× by 4×/ 4× by 4×/ 4×     by 4× 2× by 2× 5× by 5×/ 5× by     5× would 5× by 5× 5× by 5× 4.5× by     4.5×/ TEMP °C.  100° C. 100° C. 100°     C. would 100° C. 100° C. 100° C. 100° C.     140° C. 140° C. not   140° C.  b 3× by     3×   not       stretch   100° C.   mix       140° C.      c 2x by 2x/   well   100° C. FILM THICK- a = 2.5 3.0 3.5  3.0     4.0 3.0 NESS (mil) b = 6.0  c = 10.5 % AIR VOIDS a = 65 66 61   60  b =     65  c = 65 Gurley a = 0.1 sec 0.5 sec 0.8 sec  0.3 sec 1.4 sec >30 sec     85 sec >10 min  >20 sec  4.0 sec POROSITY b = 0.2 sec (METHOD B) c = 0.4     sec     *C.sub.2 C.sub.3 is an ethylenepropylene copolymer containing 2.7 mole     percent ethylene units.

All of the resulting products of the Examples were opaque white films.

Examples 1a, b, and c show that for the system LLDPE/ CaCO₃ films, 70%by weight of the filler gives much lower Gurley number (i.e., highbreathability) than 65% by weight of the filler. Examples 1a, b and cshow that breathability is the best in the more highly oriented films.Similarly, 65% by weight of the filler (Example 2 gives a much lowerGurley number than 60% filler, (Example 3). Example 4 compared toExample 1 shows that the processibility of the formulation is improvedby adding 1.5% of calcium stearate (Example 1) instead of 0.6% calciumstearate (Example 4); further, in comparative Example 4, die depositsand melt fractures were excessive and caused constant breakage of theextruded molten strands. Thus, the material could not be pelletized andextruded into castings suitable for orientation. Example 5 demonstratesthe advantage of using a small amount of polypropylene additive in theLLDPE to reduce die lines. Regions of melt fracture (die lines) thinnerthan the rest of the casting were greatly reduced compared to the meltfracture regions commonly observed in compositions such as those inExamples 1a, b and c.

Example 6 shows that, for polybutene/calcium carbonate film, 70% byweight of the filler gives much lower Gurley number (i.e., highbreathability) than 50% by weight of the filler in Example 7. Example 8,compared to Example 6, shows that 1.5% of calcium stearate allows mucheasier processing to a porous film than if no calcium stearate is used(Example 8). The film prepared in comparative Example 8 had many visiblepinholes and was extremely rough. Gurley measurements were not possible.Castings made from this composition could not be oriented 4 times by 4times at the temperature of 100° C. Example 9 shows that, forpolypropylene/calcium carbonate films, 60% by weight of the filler givesa much lower Gurley number than when only 50% by weight of the filler isused (Example 10). Example 11 compared to Example 9, shows that using ablend of polypropylene and ethylene-propylene copolymer gives betterprocessing than if pure polypropylene is substituted for the blend(Example 11) because the casting of Example 11 would not stretch to formfilm at 140° C. Example 12 shows that high calcium stearate levelsgreatly improves processibility compared to a low calcium stearate levelin Example 13 because the resulting film had large visible pin holes andwas extremely rough. Example 14 demonstrates a breathable compositioncomposed of polypropylene and glass bead filler.

EXAMPLE 15

A series of experiments were run on strawberries, mushrooms and broccolito demonstrate the reduced loss of water vapor from the produce whilenot interfering with the respiration of the produce using a containersealed with the breathable film of the instant invention. The containerwas constructed of a substantially gas-impermeable material having awindow in the top with a gas-permeable panel of a material of thisinvention therein to provide a substantially free flow therethrough ofthe O₂ and CO₂ gases yet substantially no flow of water vapor. Thedetails and results of the experiments are as follows:

    __________________________________________________________________________                            TIME IN          STEADY-STATE                                 FILM TYPE & PERMEANCE                                                                         PACKAGE (HR)                                                                            % WEIGHT                                                                             O.sub.2                                                                            CO.sub.2                        PRODUCE (g)                                                                           (cc/100 in.sup.2 -atm-day)                                                                    TEMP (°C.)                                                                       LOSS   %    %                               __________________________________________________________________________    Strawberries                                                                          Biaxial PP/60%  310 (4°)                                                                         0.1    16   6                               (652 g) Atomite. Permeance                                                            100 Million                                                           Strawberries                                                                          Control (Open To                                                                              165 (4°)                                                                         14.7   21   0                               (625 g) Atmosphere)                                                           Mushrooms                                                                             Biaxial PP/60%  145 (4°)                                                                         0.4    20   2                               (190 g) Atomite. Permeance                                                            100 million                                                           Mushrooms                                                                             Control (open)  167 (4°)                                                                         33     21   0                               (190 g)                                                                       Broccoli                                                                              Biaxial PP/60%  318 (11°)                                                                        11.5   19   3                               (247 g) Atomite. Permeance                                                            100 million                                                           Broccoli                                                                              Control (open)  318 (11°)                                                                        33     21   0                               (247 g)                                                                       __________________________________________________________________________

These experiments demonstrate that a breathable film of this inventionplaced over an aperture (window) in a substantially gas-impermeablecontainer of produce will reduce the rate of evaporation, which isbeneficial, without necessarily inducing major changes in the O₂ and CO₂levels in the package. The crispness or appearance of the strawberries,mushrooms, and borccoli was improved by the use of the breathable film.

What I claim and desire to protect by Letters Patent:
 1. A method of preparing a breathable microporous polymeric film having the composition of 20-37% by weight of a polymer or copolymer of an α-olefin having 1-8 carbons or mixtures thereof, 60-75% by weight of calcium carbonate or glass beads, 0.1-3.0% by weight of calcium stearate and, optionally, 0-2% by weight of a UV light, oxygen, and heat stabilizer by preparing a melt blend of the ingredients, maintaining the moisture level in the melt blend below 700 ppm prior to extruding a casting, and based on the polymer or copolymer used, stretching the casting in two directions of from 1.5 to 7 times in each direction in a temperature range of from about 20° to about 160° C. thereby producing the breathable microporous film having a Gurley Porosity of from 0.1 to 85 seconds so that the film has good air and water vapor transmission rates but is substantially imPenetrable by liquid water.
 2. The method of claim 1 wherein the moisture level is maintained below 300 ppm.
 3. The method of claim 2 wherein the composition has 65 to 75% of calcium carbonate.
 4. The method of claim 3 wherein the polymer is polyethylene or polybutylene and the casting is stretched in two directions from 1.5 to 5 times in each direction in a temperature range of from about 20° to about 105° C.
 5. The method of claim 3 wherein the polymer is polypropylene and the casting is stretched in two directions from 4 to 7 times in each direction in a temperature range of from about 130° to about 160° C. 